Kinetics of Rotational Motion in Liquid-Crystalline Polymers

Abstract

The rotational viscosity γ₁ of a liquid-crystalline side-group polymer depends on the temperature and the molecular mass and is always remarkably higher than for the equivalent low molecular mesogen. By measuring the rotational diffusion constant D_r of the mesogen side-groups through dielectric relaxation experiments while simultaneously determining γ₁ in director reorientation experiments, it was possible to exactly analyze the temperature and molecular mass dependence. In general, γ₁ is additively composed of a contribution γ₁^m, connected with the reorientation of the mesogen side-groups, and a second contribution Δγ₁, combined with the synchronous redirection of the main chain. As in low molecular liquid crystals, γ₁^m is in inverse ratio to D_r and is related to the glass transition temperature by the Vogel-Fulcher-Tamann law. The dependence of Δγ₁ on the molecular mass is described by a power law. Examinations were made on LC polymethacrylates of different molecular mass, as well as on LC polysiloxane-LC monomer mixtures. On the basis of an explicit equation γ₁(M,T) it is possible to give a consistent description of the measured values.